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Full text of "The manufacture and properties of iron and steel"

4                                                INTRODUCTION.
between the carbon of the gas and the iron ore, whereby the oxygen of the ore unites with the carbon and leaves the iron in the finely divided metallic state known as "spongy iron." The reaction is not complete and a great deal of ore reaches the zone of fusion in a nearly raw state, "but in this zone the extremely high temperature quickly completes all reactions; the raw ore is rapidly reduced, the earthy impurities unite with the limestone and are fused into slag, while the metallic iron melts and is collected in the hearth below the tuyeres.
The metal so produced is not pure iron, for while it is in contact with white-hot coke in the furnace, it absorbs a certain amount of carbon. This amount is quite constant, and it is safe to assume that any piece of ordinary pig-iron, no matter what its appearance may be, contains from 3.5 to 4.0 per cent, of carbon. Some of this carbon is chemically combined with the iron, and some is held in suspension as graphite. If a large proportion is combined, the fracture of the iron looks white and the metal is hard and brittle. If a large proportion is in -the free state, the fracture will be gray or black, with loose scales of graphite, and the iron is soft and tough. Very slow cooling tends to put the carbon into the condition of graphite, while sudden chilling from the liquid state tends to keep it in combination and give a hard and white iron.
The iron also contains silicon, which is absorbed in the furnace from the ash of the coke. Sometimes this silicon will amount to only one-half of 1 per cent, and sometimes it will be 3 per cent. Usually there will be from 1 to 2 per cent.
A certain small proportion of sulphur will also be present. It is not wanted at all, but there is seldom less than two-hundredths of one per cent., while there may be one-quarter of one per cent., and even more. When there is over one-tenth of one per cent, the iron is apt to be hard and brittle and to have a close and white fracture. In such iron, "the silicon is usually low and this contributes to the closeness of the grain.
The percentages of silicon and sulphur that are present in the iron depend in great measure upon the conditions in the blast furnace, and hence may be controlled by the furnaceman. ' But there is one element which is universally present, over which he has no control. This element is phosphorus. Whatever quantity is present in the ore and fuel will be found in the pig-iron, so that the only way~to get an iron low in phosphorus is to get ore and cokę